1. Field of the Invention
The present invention relates to RFID transponders and in particular reading and writing to RFID transponders using adaptive control of the RFID transceiver.
2. Description of Related Art
Radio frequency identification (RFID) transponders, either active or passive, are typically used with an RFID transceiver or similar device to communicate information from the transponders. RFID transponders are known in the art and are available in various frequencies including 860-930 MHz, 13.56 MHz, and 125-130 KHz, though this invention contemplates within its scope RFID transponders of any frequency and those that may be later developed. In order to communicate, the transceiver exposes the transponder to a radio frequency (RF) electromagnetic field or signal. In the case of a passive transponder, the RF electromagnetic field energizes the transponder and thereby prompts the transponder to respond to the transceiver by modulating the field in a well-known technique called backscattering. In the case of an active transponder, the transponder may respond to the electromagnetic field by transmitting an independently powered reply signal to the transceiver.
An interrogating electromagnetic signal is used to activate an RFID transponder, read information from an RFID transponder, and encode (write) information to an RFID transponder. Generally, read/write/activate electromagnetic signals are of fixed strength, where such fixed strength is determined empirically through a series of laboratory tests usually from a small sample of RFID transponders. If the fixed strength signal is too strong, then the RFID transponder may be physically damaged. If the signal is too weak, then the transponders may not be encoded properly or may fail to be activated for transmitting their information.
Challenges can also occur when interrogating multiple adjacent transponders regardless of whether the transponders are passively or actively powered. For example, in some applications it may be desired to only interrogate a single RFID transponder at a time, and a strong interrogating electromagnetic signal may activate more than one transponder at a given time. This simultaneous activation of multiple transponders may lead to communication, i.e. read and write errors because each of the multiple transponders may transmit reply signals to the transceiver at the same time. This is particularly problematic if the interrogating electromagnetic signal is strong and the RFID transponders are in close proximity. Furthermore, if interrogating multiple RFID transponders simultaneously, those closest to the transceiver supplying the interrogation signal may have their electronics damaged if the interrogating signal is overly strong.
It is known in the art that some RFID transponders need a more powerful interrogating signal to perform read/write/activate operations, where others need a less powerful signal. This can be attributed to, at least in part, variations in the chips used to manufacture the RFID transponders, bonding quality, contaminants, etc. This may also be the result of fundamental differences in the design of various RFID transponders. In some previous instances when a RFID transponder is not read or encoded on a first attempt, the read or encode signal strength is incrementally increased after each attempt until the read or encoding is successful or it is determined that the operation is improbable. This process requires multiple read or encode attempts and at least one power increment step and can slow down RFID transponder processing. Various other techniques of varying output power have also been employed in order to change the range of an encoding antenna and to locate the position of an RFID tag on a media strip, including a technique for locating such antenna to facilitate the throughput of RFID tags. Such techniques are described more fully in the following commonly-assigned U.S. patent applications: U.S. Patent Publication No. 2005/0274799 (Apparatus and Method for Communicating with an RFID Transponder), published on Dec. 15, 2005; U.S. Pat. No. 7,190,270 (System and Method for Detecting Transponders Used With Printer Media), issued on Mar. 13, 2007; and, U.S. Pat. No. 7,504,950 (System and Method for Continuous RFID Encoding), issued on Mar. 17, 2009, each of which are fully incorporated herein by reference and made a part hereof.
Furthermore, in some RF applications where singulated RFID transponder processing is desired, the challenge of avoiding or processing multiple transponder activation is especially troublesome. RF printer-encoders are one example. RF printer-encoders are devices capable of programming and printing a series or stream of transponders. The close proximity of the transponders and space, cost, and weight restrictions associated with such devices make multiple transponder activation problematic. Furthermore, the space, cost, and weight restrictions, among other factors, make anti-collision management techniques or shielding components for alleviating multiple transponder activation less than desirable.
In light of the foregoing it would be desirable to provide an RF system or device capable of quickly determining a signal strength for a transceiver's electromagnetic signal for efficiently reading and/or writing to an RFID transponder. It would also be desirable to provide an RF system or device capable of interrogating individual transponders positioned among multiple adjacent transponders where such system is adaptive to different transponder configurations and placements. It would also be desirable to provide an RF system or device capable of interrogating multiple adjacent transponders where such system is adaptive to different transponder configurations and placements.